1. Field of the Invention
The present invention generally relates to acoustic resonators and their applications in electronic circuits.
2. Description of the Related Art
Capacitors are a basic building block for electronic circuits. One design for capacitors is the parallel-plate configuration, in which a dielectric is sandwiched between two electrodes. FIG. 1 is a block diagram illustrating a typical metal-insulator-metal (MIM) parallel plate configuration of a thin film capacitor 100. The capacitor 100 is formed as a vertical stack comprised of a metal base electrode 110b supported by a substrate 130, a dielectric 120, and metal top electrode 110a. The lateral dimensions, along with the dielectric constant and thickness of the dielectric 120, determine the capacitance value.
Materials in the barium strontium titanate (BST) family have characteristics that are well suited for use as the dielectric 120 in such capacitors 100. BST generally has a high dielectric constant so that large capacitances can be realized in a relatively small area. Furthermore, BST has a permittivity that depends on the applied electric field. In other words, thin-film BST has the remarkable property that the dielectric constant can be changed appreciably by an applied DC-field, allowing for very simple voltage-variable capacitors (varactors), with the added flexibility that their capacitance can be tuned by changing a bias voltage across the capacitor. In addition, the bias voltage typically can be applied in either direction across a BST capacitor since the film permittivity is generally symmetric about zero bias. That is, BST typically does not exhibit a preferred direction for the electric field. One further advantage is that the electrical currents that flow through BST capacitors are relatively small compared to other types of semiconductor varactors.
Such electrical characteristics of the BST capacitors allow other potential beneficial uses of the BST capacitors in electronic circuits.